1. Field of the Invention
The present invention relates to the material, manufacture and article of abradable seals for use in gas turbine engines, and more particularly to high purity zirconia and/or hafnia based ceramic abradable seals that are thermal sprayed.
2. Description of the Related Art
Large gas turbine engines are widely used for aircraft propulsion and for ground based power generation. Such large gas turbine engines are typically of the axial type, and include a compressor section, a combustor section, and a turbine section, with the compressor section normally preceded by a fan section. Each of the fan, compressor, and turbine sections comprises a plurality of disks mounted on a shaft, with a plurality of airfoil shaped blades projecting radially therefrom. A hollow case surrounds the various engine sections. Located between the disks and projecting inward from the case assembly which surrounds the disks are a plurality of stationary vanes. During operation of the fan, compressor, and turbine sections, axially flowing gases alternately contact moving blades and the stationary vanes. In the fan and compressor sections, air is compressed and the compressed air is combined with fuel and burned in the combustion section to provide high pressure, high temperature gases that flow through the turbine section, where energy is extracted by causing the bladed turbine disks to rotate. A portion of this energy is used to operate the compressor section and the fan section.
Engine efficiency depends to a significant extent upon minimizing leakage by control of the gas flow in such a manner as to maximize interaction between the gas stream and the moving and stationary airfoils. A major source of inefficiency is leakage of gas around the tips of the compressor blades, between the blade tips and the engine case. Accordingly, means to improve efficiency by reduction of leakage are increasingly important. Although a close tolerance fit may be obtained by fabricating the mating parts to a very close tolerance range, this fabrication process is extremely costly and time consuming. Further, when the mated assembly is exposed to a high temperature environment and high stress, as when in use, the coefficients of expansion of the mating parts may differ, thus causing the clearance space to either increase or decrease. The latter condition would result in a frictional contact between blades and housing, causing elevation of temperatures and possible damage to one or both members. On the other hand, increased clearance space would permit gas to escape between the compressor blade and housing, thus decreasing efficiency.
One means to increase efficiency is to apply a coating of suitable material to the interior surface of the compressor and/or turbine housing, to reduce leakage between the blade tips and the housing. Various coating techniques have been employed to coat the inside diameter of the compressor and/or turbine housing with an abradable coating that can be worn away by the frictional contact of the blade, to provide a close fitting channel in which the blade tip may travel. Thus, when subjecting the coated assembly to a high temperature and stress environment, the blade and the case may expand or contract without resulting in significant gas leakage between the blade tip and the housing. This abradable coating technique has been employed to not only increase the efficiency of the compressor/turbine, but to also provide a relatively speedy and inexpensive method for restoring excessively worn turbine engine parts to service.
Increased firing temperatures is another approach to improved engine efficiency. However, even nickel and cobalt superalloys are not capable of surviving long term operation at the firing temperatures of modern gas turbine engines that may exceed 1,400° C. in oxidizing environments. In order to provide additional protection to the metal components in the hottest areas of a gas turbine engine, it is known to coat the metal substrate with a layer of ceramic material to thermally insulate and chemically isolate the substrate from the hot combustion gasses. A widely used material for this application is yttria stabilized zirconia (YSZ), with 6-9 weight percent yttria (Y2O3) being a common composition.
Technical benefits of 6-9 weight percent YSZ include a high thermal expansion coefficient compared to other ceramics such as alumina. Typically the expansion coefficient mismatch is more comparable to superalloy based materials and oxidation resistant bond coat alloys. Other technical benefits of 6-9 weight percent YSZ include excellent thermal insulation, the fact that it is chemically inert for most environments, thermally stability up to 1200° C. under isothermal or cyclic conditions, and general ease and cost effectiveness of application through thermal spray technology.
Besides material properties, microstructure plays an important role in engine performance. An important consideration in the design of a thermal barrier is that the coating be a ceramic material having a crystalline structure containing beneficial cracks and voids (porosity), imparting strain tolerance. If there were no cracks in the coating, the thermal barrier would not function, because the differences in thermal expansion between the metal substrate system and the coating will cause interfacial stresses upon thermal cycling that are greater than the bond strength between them. By the creation of a crack network into the coating, a stress relief mechanism is introduced that allows the coating to survive numerous thermal cycles. Repeating crack networks are typically imparted into the coating on varying space scales by manipulating the thermodynamic and kinetic conditions of the manufacturing method, and different structures known to perform the coating task have been optimized likewise. In addition to this, cracks are also formed during service, so the structure formed upon coating manufacture changes with time, depending on the starting material phases in the manufactured coating and thermal conditions during service. There remains a need in the art for a coating material, coating material manufacturing method and coating manufacturing method that address the changes in the coating microstructure during its service lifetime.
Besides microstructure characteristics of YSZ coatings, another design factor determining coating lifetime is the sintering rate of the coating. When the coating is cycled above half of its absolute melting temperature, the coating begins to sinter causing volume shrinkage. As the coating shrinks, the stress difference between the coating and substrate increases. At a certain amount of shrinkage (which varies depending on the type of structure and thermal conditions during service), the stress difference exceeds the bonding strength of the coating and it becomes detached. Decreasing the sintering rate of the thermal barrier increases the amount of time before the catastrophic shrinkage is experienced, so it can become a major design consideration. For high purity zirconia alloys, the onset of sintering commences at temperatures above 1000° C.
The thermal insulating properties of ceramic thermal barrier coatings (TCB) have been the subjects of many design improvements over the years. Known thermal barrier coating materials include the use of zirconia stabilized with both yttria and erbia (Er2O3) in designated amounts. Other thermal barrier coating materials include gadolinia and hafnia, preferably forming gadolinia-hafnia, and a zirconia-based material with a dysprosium oxide having the dual function of stabilizing the zirconia and reducing the thermal conductivity of the zirconia due to phonons.
Abradable thermal barrier coatings require a highly porous coating network of, typically, between 20-35 percent porosity, which cannot be achieved by conventional flame spray techniques. Porosity is needed in order for the turbine blades to cut grooves in the abradable coating. Previous testing of ceramic materials has shown that high porosity levels, in excess of about 35 volume percent, produce coatings prone to erosion damage. Porosity levels of less than about 20 volume percent are unsatisfactory because they cause excess blade tip wear. The material from which the coating is formed must abrade relatively easily without wearing down the blade tips. This requires a careful balance of materials in the coatings. In this environment, an abradable coating must also exhibit good resistance against particle erosion and other degradation at elevated temperatures. As known by those skilled in the art, however, these desirable characteristics have been difficult to obtain. The porosity levels achieved by conventional techniques for ceramic coatings using conventional powders normally range between 5 and 20 percent, and the porosity level, it has been found, is a direct function of the powder size and spraying parameters, e.g., spray rate, spray distance and power levels of the spray gun.
Past turbine sealing structures have taken a variety of forms. Some of the currently favored approaches include complex plasma sprayed structures that vary in composition from metal at one surface to ceramic at an outer surface with variations in composition, stress and porosity in between. These structures usually have a thickness on the order of 4 mm and are costly because of the need to carefully control the substrate temperature and plasma spray conditions, during the deposition of many layers, to achieve the correct abradable and durable structure. Such thick seal structures will spall and fail if the deposition parameters are not followed closely. Likewise in service these seals with their built-in varying stresses are subject to foreign object damage. When failure of thick seals does occur excess leakage results through the resulting wide gap.
With continuing efforts to improve gas turbine efficiency through use of better seals and use of higher temperatures, there remains a need in the art for coating materials and coating application methods that provides improved high-temperature properties and wear characteristics for abradable thermal barrier coatings.